In light of a decrease in electric power consumption of home electric appliances, an emphasis has been placed on reducing power loss of semiconductors such as power devices and power modules. In view of the circumstances, semiconductor devices such as power devices employing wide bandgap semiconductors have been developed for the purpose of further reduction in power loss.
Since field-effect transistors (FET), which are made of the III group nitride semiconductor of a GaN (gallium nitride) system, are expected to bring about low on-resistance, various semiconductor devices employing such field-effect transistors have been developed. A known example of the field-effect transistors of a GaN (gallium nitride) system is an AlGaN/GaN HFET (Heterojunction FET) employing a heterostructure in which GaN and AlGaN (aluminum gallium nitride) are combined. Since the AlGaN/GaN HFET has characteristics such as low on-resistance, high-speed performance, high withstand voltage, and high resistance to heat, the AlGaN/GaN HFET has become a focus of attention.
However, the AlGaN/GaN HFET is typically a normally-on field-effect transistor, and is therefore in an on state while a gate voltage is 0V. In order for the AlGaN/GaN HFET to be put in an off state, negative voltage needs to be applied to a gate. Note that, since a power device handles a vast amount of electric current, the power device is required to employ a normally-off field-effect transistor which blocks an electric current while zero bias voltage is applied. This causes a problem that a normally-on field-effect transistor such as the AlGaN/GaN HFET cannot, without any alterations thereto, be used in a power device.
In view of the problem, Patent Literature 1 discloses a semiconductor device in which a normally-off operation is realized by employing a normally-on active element. According to the semiconductor device, a normally-on field-effect transistor and a normally-off field-effect transistor are cascode-connected to each other.
Specifically, according to the semiconductor device, (i) a drain of a first field-effect transistor of a normally-on type is connected to a first node, (ii) a source of a second field-effect transistor of a normally-off type is connected to a second node, and (iii) a switching element is provided between a control electrode of the first field-effect transistor and the second node. While the semiconductor device is in an off state, the switching element operates, by causing the first field-effect transistor to be in an on state, so as to maintain a voltage less than a withstand voltage of the second field-effect transistor. In a case where a voltage across a control electrode reaches or rises higher than a threshold voltage, the switching element allows electric conduction from the first node to the second node.
The semiconductor device configured as such operates as a normally-off transistor which is turned on by applying positive voltage to the control electrode of the second field-effect transistor. Then, by sealing such a semiconductor device in a package such as TO220, it is possible to provide a normally-off field-effect transistor which is high withstand voltage and which is capable of a high-speed operation.